1. Field of the Invention
The present invention generally relates to a two-cycle combustion engine having an air scavenging system, which can be utilized as a drive source for a small-size work machine such as, for example, a brush cutter or a blast cleaner. More particularly, the present invention relates to the two-cycle combustion engine of the type referred to above, having a pressure reducing device for discharging compressed gases within an engine cylinder.
2. Description of the Prior Art
The two-cycle combustion engine having an air scavenging system is well known in the art. Specifically, the conventional two-cycle combustion engine with the air scavenging system is generally so designed that prior to a combustion chamber being scavenged with an air-fuel mixture, the combustion chamber is initially scavenged with an air to suppress a blow-off of the air-fuel mixture through an exhaust port. In this conventional two-cycle combustion engine, for example, a pair of first scavenging passages and a pair of second scavenging passages are defined in part in the engine cylinder and in part in the crankcase, respectively, such that the air is temporarily introduced into the second scavenging passages and is subsequently supplied into the combustion chamber through the second scavenging passages prior to the air-fuel mixture being supplied into the combustion chamber through the first scavenging passages during the scavenging stroke. See, for example, International Publication No. WO2004/038195. According to the known construction described above, the air introduced into the combustion chamber through the second scavenging passage can effectively prevent the air-fuel mixture, once supplied through the first scavenging passage, from being blown off through the exhaust port.
Also, in order to reduce the driving torque necessary to start the combustion engine, Japanese Laid-open Utility Model Publication No. 6-14453, published Feb. 25, 1994, suggests the use of a decompressing groove that is defined in the wall of the cylinder block for driving the air-fuel mixture, compressed within the engine cylinder, out to the scavenging passage.
It has, however, been found that in the two-cycle combustion engine with the air scavenging system such as disclosed in the first mentioned Japanese publication, although both the amount of unburned gases being blown off and the fuel consumption are rendered small, the concentration of the air-fuel mixture within the engine cylinder tends to be lowered as a result of the scavenging with air, accompanied by lowering of the cooling performance exhibited by a fuel component and, therefore, the combustion chamber will not be sufficiently cooled when, for example, the combustion engine is abruptly decelerated from a fully loaded operating condition down to the idling speed. Due to such insufficient cooling of the combustion chamber, even after the ignition switch has been turned off to halt the engine, the temperature of the air-fuel mixture in the combustion chamber at the time of the start of compression remains high, resulting in the “run-on (or self ignition)” phenomenon, in which the combustion engine continues to fire. In view of the foregoing, it may be contemplated to employ the decompressing groove such as disclosed in the second mentioned Japanese publication to reduce the pressure in the combustion chamber during a low speed operation. However, it has been found that in this decompressing groove, a quantity of fuel deposited in the vicinity of an exit end thereof may be charred to form a carbon deposit, which constitutes a cause of clogging occurring in the vicinity of the exit end of the decompressing groove to such an extent as to result in closure of the decompressing groove. Once the decompressing groove is so closed, there is a high possibility that the pressure reducing effect may be lowered. Also, since the position at which the decompressing groove can be formed is limited to a site around the scavenging port, complicated and time-consuming procedures would be required to clean off the deposit clogging in the vicinity of the exit end of the decompressing groove.